The Constructor

Fundamental Assumptions for Reinforced Concrete Design

Fundamental Assumptions for reinforced concrete behaviour

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The fundamental assumptions for reinforced concrete design permit one to predict by calculation the performance of reinforced concrete members. Of course, the structural mechanics in conjunction with extensive testing are used as a base for the design and analysis of concrete structures.

The fundamental assumptions of reinforced concrete design are the basis for the structural mechanics which is one of the main tools used for the analysis and design of reinforced concrete elements.

Design, which is the main task of a structural engineer, is the determination of the general shape and all specific dimensions of a particular structure so that it would perform the function for which it is created and safely withstand the influences that act on it throughout its useful life. 

Fundamental Assumptions for Reinforced Concrete Behavior

The fundamental propositions on which the mechanics of reinforced concrete is based are as follows:

1. The internal forces, such as bending moments, shear forces, and normal and shear stresses, at any section of a member are in equilibrium with the effects of the external loads at that section.

2. Sections perpendicular to the axis of bending, that are plane before bending remain plane after bending.

3. It is assumed that perfect bonding exists between concrete and steel at the interface so that no slip can occur between the two materials.  Hence, as the one deforms, so must the other. With modern deformed bars, a high degree of mechanical interlocking is provided in addition to the natural surface adhesion, so this assumption is very close to correct.

4. In view of the fact that the tensile strength of concrete is only a small fraction of its compressive strength, the concrete in that part of a member which is in tension is usually cracked. While these cracks, in well-designed members, are generally so narrow as to be hardly visible (they are known as hairline cracks), they evidently render the cracked concrete incapable of resisting tension stress.  Correspondingly, it is assumed that concrete is not capable of resisting any tension stress whatsoever.

The theory is based on the actual stress-strain relationships and strength properties of the two constituent materials.

Fig. 1: Plane Section Remains Plane After Bending
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